Backflow recovery propeller device



1970 HIDETSUGU KUBOTA 3,549,271

BACKFLOW RECOVERY PROPELLER DEVICE Filed 001;. 2, 1968 2 Sheets-Sheet l2 I 30 k 3b4 IN VIL'N'I I IR Dec. 22, 1970 HIDETSUGU KUBOTA BACKFLOWRECOVERY PROPELLER DEVICE 2 Sheets-Sheet 2 Filed Odt. 2, 1968 Nh'YIHDIS'ISUUU KUBO'IA A'ITOR United States Patent BACKFLOW RECOVERYPROPELLER DEVICE Hidetsugu Kubota, 122 Kaizuka, Kameda-rnachi,Naka-Kanbara-gun, Niigata-ken, Japan Filed Oct. 2, 1968, Ser. No.764,575

Claims priority, application Japan, Oct. 12, 1967,

42/ 65,551 Int. Cl. B64c 11/48 US. Cl. 416124 1 Claim ABSTRACT OF THEDISCLOSURE A propeller device comprising a fixed main propeller and afreely rotatable backflow recovery propeller, both operated in the samedirection on a common shaft. The recovery propeller has a collisionsurface for the backflow current of a high pitch and the oppositesurface thereof is effectively of a low pitch. These two surfacesconverge into a semicircular shaped leading edge whereby the bladethickness ratio of the backflow propeller progressively increasestowards the end of the blade. Such a construction can substantiallyconvert the backflow energy into a recovered propelling force.

The present invention relates to a backflow recovery propeller device.

An object of the present invention is to provide a propeller device inwhich the inertia of the twisted fluid flow being produced by the fixedmain screw propeller is substantially utilized.

Another object is to provide a freely rotatable recovery screw propellerhaving a collision surface for the backflow current of a high pitch andthe opposite surface thereof is thus effectively of a low pitch.

Other objects and advantages of the present invention Will be apparentfrom the following description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a side view of an embodiment of the present invention,

FIG. 2 is a front view, and

FIG. 3 is a section of blade thickness ratio taken along line IIIIII ofFIG. 1, showing a variation of blade thickness.

Referring to the drawings, 1 shows a propeller shaft, 2 a fixed mainscrew propeller, 3 a freely rotatable recovery screw propeller, 9 a cap,A is a direction of rotation and D a distance or gap between the mainpropeller 2 and the freely rotatable recovery propeller 3. 2a and 2bshow leading and trailing edges of the main blade. 3a and 31) show alsoleading and trailing edges of the recovery blade. 3a and 3b show thesmallest and the largest recovery blade thickness portion, the formerbeing in a zone of stationary fluid and the latter in a zone of therotating flow or twisted flow. Sal shows the end portion of the smallestthickness blade. 3171 and 3172 show the end portions of the inner blade.3b3 shows a colliding surface. 3174 shows a semicircular shaped portionfor the leading edge.

The recovery propeller device according to the present inventionconsists of the two screw propellers 2, 3 mounted on the shaft 1. Thecontour of the blade of the conventional main screw propeller 2 is madesimilar to the substantially straight line of the leading edge 3a of theadjacent recovery screw propeller 3. The trailing edge 2b of the mainscrew propeller 2 is made in a substantially straight line and slightlyspaced from the leading edge 3a of the recovery screw propeller 3. Themain screw propeller is fixed on the propeller shaft 1 and operates as aconventional type blade propeller. The freely rotatable recovery screwpropeller 3 adjancent the main propeller 2 3,549,271 Patented Dec. 22,1970 is movably mounted on bearings on the shaft 1. It is to be notedthat a hub, in both sides of which sea water is prevented from flowingin by special oil seals. The lubrication thereof is eifected by a simplelubricating hydraulic system or by a sea water lubrication system havinga special nonabsorbing synthetic resin.

The feature of the recovery blade includes making both sides of ahelical configuration and thus forming a colliding surface 3113 forreceiving the twisted backflow. Its opposite surface, i.e., a propulsionproducing advance surface, or a backflow colliding surface is made in asubstantially straight line. Thus the structural and functionalcombination of both sides renders blade thickness ratio that is small atthe root, large at the blade end; that is, increasingly large inside thezone defined by the diameter of the main screw propeller 2 and suddenlydecreases to an extreme at a zone extended from the boundary defined bythe diameter of the main blade. The pitch of the advance surface of therecovery blade may preferably be about five times large as that of themain blade. This relatively high pitch ratio is theoretical but isreasonable.

The present invention comprises a backflow recovery screw propeller inwhich the opening ends of the helical surface of the advance surface andthe helical surface or plain surface of the backflow colliding surfaceconverged into a semicircular shaped leading edge. The helical surfacesof the left and right sides thus have elevation angles that are combinedinto one blade. A twisted backflow torque is absorbed by the backflowcolliding surface to produce a rotating force. The driving surfacerenders a high pitch while the advance surface a low pitch so as toobtain an effective propulsion force. As described above the recoveryscrew propeller is arranged to form in a zone in the twisted rotatingflow that is defined by the diameter of the main screw propeller and isarranged to have the smallest in blade thickness within the stationaryfluid. Consequently the main blade produces an effective propulsionforce.

If a conventional main screw propeller having blades of a thin thicknessis used as the secondary freely rotating screw propeller, the propulsiveforce is substantially cancelled by the thrust produced opposite to thepropelling direction which is produced on the backflow collidingsurface. The secondary propeller will be thus moved in the oppositedirection, even if the pitch ratio thereof is about two times that ofthe main propeller. In this case, efficiency will increase somewhat atabout the slip ratio, but since the efficiency will be down, the use ofsaid secondary propeller is virtually impossible at the normal slipratio. The secondary propeller with thin blades is obviously small inblade thickness ratio and carries away backward. There is produced anopposite thrust substantially cancelling the propulsive thrust on therotating advance surface and definitely does not increase theefliciency.

-In the propeller device according to the present invention, theeffective resistance is produced by mutual collision of the fluidbetween both propellers and the energy which would be lost in thebackflow is substantially recovered. With decreasing energy transmittingin the fluid, total propulsion efliciency is raised. The rotatingdirection of the main screw propeller and of the recovery screwpropeller are the same. In order to absorb the maximum momentum of thetwisted flow produced by the main screw propeller the driving of thescrew propeller is obtained entirely by the very high pitch. The advancesurface of the screw propeller blade producing the more effective thrustis given the necessary rotating power factor to absorb entirely thebackflow torque. This is suitable in the /8%() rotating rate of the mainscrew propeller. In order to determine the blade area and the pitch,both sides converge, as described above, on the opening in the backflowcolliding surface and the leading edge constitutes a semicircular shapedportion. The blade thickness ratio is made small atthe root and large atthe blade end.

As the both propellers rotate in same direction, the effects of themutual relationships of the blades affect the lattice effect occur. Thenasthe efliciency rises, the pitch factor in hydrodynamics become high.Thus propelling efliciency is raised in a zone of low factor inhydrodynamics. There is no effect of the twisted rotating flow in thezone outside the diameter of the main propeller. It is not necessary tocope with the eifect of the complicated fluid flow produced by thepropeller, only the advance surface by virtue of its very thin bladethickness. While the twisted backflow produced by the main propeller ispushed at a right angle to the working surface of the main blade,

,such flow collides with thebackflow colliding surface of the same butat a higher pitch. The freely rotating recovery blade is thus rotated bycollision of the fluid, and

then some slip does occur. Fluid collision with a rebound of slip fluidis continuously produced between the both propellers. Such state is theeffective resistance that occurs atthe back of the propeller andproduces a state of high pitch. That is, the quantity of kinetic energy.transmitting fluid through the main blade decreases and the low pitchpropelling efficiency increases in the main blade. Total efiiciency ofthe main blade efiiciency and blackflow. recovery amounts to about20-30% in propelling efliciency compared with the conventional mainpropeller.

In order to obtain the highest eificiency for the pitch for the recoverypropeller, a pitch. of about five times larger than as that of the mainpropeller in a slip ratio situation of about.20% for the propellerdevice, a pitch is selected of about four times larger than of the mainpropeller; In a slip ratio of about a pitch is selected of about"3.5times larger than that of the main propeller. In slip ratio about Ofcourse blade area ratio and the like must be considered.

According to a feature of the present invention, the change ofbladethickness ratio shownin FIG. 1 makes a colliding surface of .thebackflow a surface coincided with the advance surface, plain surface inaxial direction or the helical surfaces of both left and right sides.The

blade thickness ratio is made the smallest, considering that in a zoneoutside the diameter of the main blade there in no substantial effect onthe backflow. The advance propelling surface can determine a pitch bythe backflow recovery and thus a slip ratio of the highest eificiency ofthe propeller is achieved. The reason why the leading edge of therecover blade propeller has a semicircular configuration and' not atapered aerofoil is that if an aerofoil is used, the backflow collidingsurface becomes increasingly large and flows in the opposite directionto produce a harmful resistance.

I claim:

1. A propeller device comprising a power operated shaft having a hubportion, a main screw propeller axially and rigidly mounted on saidshaft, a backflow recovery screw propeller axially positioned adjacentsaid main propeller and mounted to rotate relatively free on said shaft,a cap member axially positioned adjacent said recovery propeller andmounted on one end of said shaft, said main and recovery propellersoperatively rotated in the same direction, said recovery propellerhaving helical shaped surfaces on both sides thereof, said sidesconverging to form a leading edge having a semicircular configuration,said recovery propeller having a thickness in the longitudinal directionthat progressively increases more than one half of the longitudinaldistance from the hub portion to the propeller end, the longitudinaldistance nearest the end of the propeller having a constant uniformthickness of substantially less than any of the progressively increasingportion whereby the structure of the recovery propeller willsubstantially convert the backflow force produced thereby into a forwardrecovered propelling force.

References Cited UNITED STATES PATENTS 2,126,221 8/ 1938 Sessums -13525FOREIGN PATENTS 332,124 7/1930 Great Britain 170-13525 91,268 10/1921Switzerland 170-135.25

EVERETTE A. POWELL, 111., Primary Examiner

